The invention relates to an extension device for a bone anchor, in particular for a bone screw, in particular for a pedicle screw, in particular in minimally invasive spinal surgery, having an axial longitudinal direction, a circumferential direction concentric thereto and a radial direction, wherein the extension device is extended in the axial longitudinal direction and can be detachably fixed in the longitudinal direction to the head of the bone anchor via a radial projection of an engageable area on the head of the bone anchor in the longitudinal direction and also engages in the radial direction and is detachable at the head of the bone anchor, but is rigidly and also rotationally fixed in the longitudinal direction,
wherein the extension device comprises two shell parts that are separated from each other and extended in the longitudinal direction, each of which comprises a radial projection for engaging behind the engageable area on the head of the bone anchor,
wherein the respective shell part (22) is predominately designed to be dimensionally stable. A device of this type is known from the not-previously published patent application DE 10 2017 114273.8 of the applicant.
Extension devices of the type named above are preferably used in minimally invasive procedures, in particular for implantation of bone anchors. They are intended to keep a working channel to the bone anchor accessible during the surgical procedure, through which the surgeon can attach the bone anchor by means of additional instruments, and, if necessary, supply and attach additional implant parts and implement repositioning measures.
An extension device having two shell parts separated from each other is known, for example, from WO 2007/021588 A1. These shell parts must be attached by means of an additional mounting part on the head of the bone anchor that connects the two parts. A radial engagement of the shell parts on the head of the bone anchor does not seem to be provided.